Having two or more sets of dice in different colors allows you to roll more things at once while still being able to differentiate between rolls. This can make the game move signicantly faster.

Having a few extra dice bags that each contain two or more sets of dice means that you're ready for new players who don't have dice of their own.

Now remember that that there are many games out there, and they don't all use the same dice. Better get several bags of dice for each game.

And that's why I've got a sack of 120 d12s here.